1. Field of Invention
The present invention relates to a microwave heating oven which heats a material by applying microwave. More particularly, it relates to an improvement for heating uniformly a material at high temperature.
2. Description of the Prior Arts
A microwave heating oven has been used for preparing a sintered product of ceramic, earthen ware or a metal oxide having zinc oxide as a main component, etc.
The oven shown in FIG. 1 has been known as said microwave heating oven.
In FIG. 1, a microwave generator (1) is coupled through a waveguide (2) to a microwave oven (3). A heating material (4) is placed on a supporting bed (7) in the microwave oven (3).
In said structure, the microwave generated by the microwave generator (1) is applied through the waveguide (2) to the microwave oven (3). Resonance electromagnetic field having various modes is formed in the microwave oven (3). When a material being heated (4) is placed in the microwave oven (3), the material (4) is heated by dielectric heating caused by the microwave.
Thus, uneven heating is caused for the material (4) in the conventional microwave oven because of uneven electromagnetic field. Moreover, it has been difficult to heat the material (4) at a temperature required for sintering a ceramic such as higher than 1000.degree. C. because of heat radiation caused by radiation and natural convection. For example, a black sperical ball having a diameter of 7 cm is used as the material (4). The heat radiation caused by radiation at 1000.degree. C. is given by the equation: ##EQU1## The heat radiation caused by natural convection is given by the equation: ##EQU2## wherein the heat transfer coefficient is 10 k cal/m.sup.2 .multidot.hr.multidot.deg.
Therefore, the microwave input having greater than 2 kW has been required.
When the material (4) is a solid, it has been difficult to perform suitable impedance matching whereby heating efficiency has been disadvantageously low.